1. Field of the Invention
The present invention relates to electrodeposition of aqueous-based coating compositions onto electroconductive substrates. More particularly, the invention relates to a method of continuously electrocoating a length of flat metal sheet such as a continuous length of metal coil.
2. Brief Description of the Prior Art
Coal coating involves the coating of a continuous length of flat metal sheet. The sheet which is usually thin gauge steel or aluminum is usually coiled over a spool which is continuously unwound and passed to a coating station where the sheet is coated in a continuous manner as it passes through the station. At the coating station, which is usually a roll coater or spray coater, a primer coat or a top coat is applied and the coated substrate is then passed to a baking oven for curing. If a primer coat is applied, a top coat is applied after baking of the primer at a second coating station. The top coat must then also be subsequently baked and cured.
There are a number of disadvantages associated with this coating system. In applying only a top coat with no primer coat, the metal substrate does not have outstanding corrosion resistance due to relatively poor adhesion to the substrate and pinholes in the coating. These problems can be overcome by applying a primer coating. However, conventionally applied primers must be baked before application of the top coat. If not baked, the primer will be wet and tacky and will stick to the conveyor rolls transferring the coated coil strip to the topcoater. Also, if the primer coat is wet, it may cause the top coat to blister in the subsequent baking operation. Although the primer coat can be baked before topcoating, this requires an additional baking oven with attending high equipment costs and energy consumption.
It is therefore an object of the present invention to provide a coil coating process which overcomes the above-mentioned difficulties. It is an object of the present invention to provide a coal coating process in which a primer and top coat can be applied sequentially and continuously to a length of metal coil and the two coatings baked and cured simultaneously to produce the final coated article.